In recent years, with the advent of microelectronics circuitry and related advances in electrical engineering, many industries have found a greater need to non-invasively measure the electrical and magnetic properties of materials and devices. The process of magnetic imaging at high spatial resolution and high sensitivity has been impractical, while low sensitivity or low spatial measurements have been unable to resolve crucial electrical properties.
In the field of semiconductors/microelectronics testing, there is a need to measure the current flow and image the data relating to the operation of semiconductor/microelectronic devices and their related current paths.
With the advent of magnetic resonance imaging in the field of biology, many new discoveries have been made regarding biological and biochemical subjects. Unfortunately, none of the current technologies applied in this field provide a very sensitive reading in the picotesla range at low frequencies, or provide good spatial resolution at high frequencies.
A number of techniques have been developed to image magnetic fields at length scales of a few .mu.m or relatively smaller. These include decoration techniques, magnetoresistive or Hall probe sensors, magneto-optic thin films, magnetic force microscopy, and electron beam interferometry. These techniques have provided limited success and are not practical for high resolution and high sensitivity imaging of fields and flux lines.
Additionally, a number of susceptometers and magnetometers have been proposed using Superconducting Quantum Interference Devices, or SQUIDS. Though previous SQUID systems have been developed to provide high magnetic field resolution, they are impractical to implement in a microscope imaging device. The prior art magnetic imaging devices using SQUIDS have had spatial resolution on the scale of a mm or larger which is too crude for microscopically resolving images. These devices may also require placing samples in a vacuum. Of course, many samples such as liquids and biological specimens cannot tolerate a vacuum. Thus it is not practical to measure sources of biomagnetism which are currently the focus of much of the existing low-spatial-resolution SQUID imaging work.
U.S. patent application Ser. No. 08/061,102, now U.S. Pat. No. 5,491,411, entitled "Method and Apparatus for Imaging Microscopic Spatial Variations in Small Currents and Magnetic Fields," by Wellstood et al., herein incorporated in its entirety by reference, discloses one such apparatus capable of providing all of the above discussed measurements with enhanced spatial resolution and magnetic field sensitivity. However, the device still requires placing a sample within a dewar, which may result in the unwanted destruction of the sample when it is exposed to the cryogenic liquid or vacuum. Even if the sample could tolerate the vacuum environment or cryogenic environment, it is time consuming and cumbersome to introduce a sample into a vacuum space for imaging. Another shortcoming is the limited size of the samples that can be imaged.